1. Field of the Invention
This invention relates to a navigation system installed in a movable body such as an automobile, and more particularly to a navigation system for a movable body wherein a map as well as a current position, a locus of movement and so forth of an automobile are displayed on a display unit such as a cathode ray tube to provide driving information to a driver.
2. Description of the Prior Art
A navigation system installed in a movable body, such as an automobile, is generally constituted such that a map of the whole of country or a plurality of divided regional maps are stored as map information in map storage means. The map information is read out in accordance with a demand of an operator and displayed on a display unit such as a cathode ray tube to provide a map for driving to the operator.
Further, a current position of a vehicle is determined by a calculation based detection signals from a distance sensor for detecting a travel distance of the vehicle and a direction sensor for detecting a driving direction of the vehicle. A triangle mark indicative of the current position of the vehicle and a plurality of serial dots indicative of a driving locus are displayed on a map displayed on the display unit so that the position of the vehicle on the map on the screen may be visually discerned. Such navigation system is disclosed, for example, in Japanese Patent Laid-Open No. 53711/1983, Japanese Patent Publication No. 39800/1984 and Japanese Patent Laid-Open No. 165511/1985.
With a system of the type described above, however, while a locus of movement of the vehicle can be displayed in an overlapping relationship on a map displayed on the screen, it is impossible to arbitrarily erase the locus of movement. Accordingly, when the locus of movement is overlapped with one of line segments defining a road, a bridge or the like of the map displayed on the screen during driving of the vehicle, it is often difficult to identify the locus of movement from the line or the locus may be mistaken as the line. Thus, the locus of movement displayed is normally somewhat confusing, and wrong information may be provided to the driver.
Further, in such conventional navigation system as described above, when a locus of movement is to be displayed on the screen, position data are calculated and successively stored into storage means at predetermined intervals of time. Thereafter, upon display of a map, such position data are read out from the storage means and displayed in the form of a plurality of serial dots on the screen.
It is to be noted that, since a map displayed on the screen is limited, when the locus of movement to be displayed on the screen exceeds the limit, setting of a new starting or reference point is effected by means of an operating device to establish a continued display of driving of the vehicle.
While a system of the type described above requires a comparatively small memory capacity because position data are stored intermittently at intervals, since position data are stored into a single locus memory after each fixed interval of time, dots displayed on the screen and indicative of a locus of movement are spaced from each other by a fixed distance. Consequently, even when a map displayed on the screen is replaced by another map having a different scale, for example, when a wide area map is replaced by a detailed map, the distance between dots of the locus of movement is maintained, and consequently, the driving locus is not displayed correctly in accordance with the new map of the different scale. Accordingly, the locus of movement to the current position cannot be visually discerned correctly.
Further, each time the driving locus displayed on the screen comes to a limit of the display, setting of a new starting point must be effected. Such setting operation, however, is very cumbersome during driving of the vehicle.